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This item represents the gross loss realized on the disposal of held-to-maturity securities during the period.

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No definition available.

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This item represents the gross profit realized on the sale of other investments.

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No definition available.

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This item represents the gross loss realized on the sale of other investments.

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This item represents the gross loss realized on the sale of short-term investments.

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No definition available.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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-Name Accounting Standards Codification

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Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
HTM fixed income securities gains									$ 2,000	$ 195,000	$ 194,000
HTM fixed income securities losses									(20,000)	(95,000)	(217,000)
Short term investments realized losses									0sigi_ShortTermInvestmentsRealizedLosses	0sigi_ShortTermInvestmentsRealizedLosses	(2,000)sigi_ShortTermInvestmentsRealizedLosses
Other investment securities gains									1,000sigi_OtherInvestmentsRealizedGains	0sigi_OtherInvestmentsRealizedGains	1,000sigi_OtherInvestmentsRealizedGains
Other investment securities losses									0sigi_OtherInvestmentsRealizedLosses	(1,060,000)sigi_OtherInvestmentsRealizedLosses	(400,000)sigi_OtherInvestmentsRealizedLosses
Net realized investment gains	(389,000)us-gaap_GainLossOnSaleOfSecuritiesNet	15,231,000us-gaap_GainLossOnSaleOfSecuritiesNet	4,539,000us-gaap_GainLossOnSaleOfSecuritiesNet	7,218,000us-gaap_GainLossOnSaleOfSecuritiesNet	(1,208,000)us-gaap_GainLossOnSaleOfSecuritiesNet	13,431,000us-gaap_GainLossOnSaleOfSecuritiesNet	5,154,000us-gaap_GainLossOnSaleOfSecuritiesNet	3,355,000us-gaap_GainLossOnSaleOfSecuritiesNet	37,703,000us-gaap_GainLossOnSaleOfSecuritiesNet	26,375,000us-gaap_GainLossOnSaleOfSecuritiesNet	13,252,000us-gaap_GainLossOnSaleOfSecuritiesNet
Proceeds from sale of available-for-sale securities									259,000,000	135,900,000	205,300,000
Realized Gains On Equity Portfolio Related To Change In Strategy [Member]
Schedule of Available-for-sale Securities [Line Items]
AFS securities gains									9,200,000
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AFS securities gains									1,945,000	3,340,000	4,452,000
AFS securities losses									(392,000)	(373,000)	(472,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AFS securities gains									36,871,000	24,776,000	10,901,000
AFS securities losses									$ (704,000)	$ (408,000)	$ (1,205,000)